1. Technical Field
The present invention relates generally to the field of data transmission, and in particular to efficient receiver equalization techniques. Still more particularly, the present invention provides power and area efficient high performance equalization scheme for reliable high-speed data transmission.
2. Description of the Related Art
High-speed serializer/deserializer (SERDES) technology has been under active development in the recent 20 years and grows into a multi-billion dollar industry. Nowadays, this technology has been widely used in data storage systems, telecommunications, computer industries and many other fields. The desire for higher transmission bandwidth and speed never stops. Signal transmission involving one transmitter (TX), one pair of transmission media, and one receiver (RX) constitutes a single lane transmission. Ten years ago, people were struggling with designs reaching single lane transmission of 2 Gigabits per second (Gbps) in complementary metal oxide semiconductor (CMOS) technology. Presently, that yard-stick has passed 10 Gbps.
Many legacy backplanes were originally designed for lower speed transmission of less than 3 Gbps. However, there exists eminent commercial value in deploying these backplanes for higher speed data transmission. A backplane is a circuit board, usually a printed circuit board, which connects several connectors in parallel to each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus. To adapt the existing backplanes for higher speed data transmission, many challenging technical issues must be solved. One issue involves channel non-idealities, which are characteristics of the transmission media that attenuate the transmitted signals. At data rate of 6 Gbps and above, the channel non-idealities causes signal loss, reflections, as wells as significant high frequency crosstalk. Signal loss is the loss of the strength of a signal. Another technical issue is signal reflection, which occurs when a signal is transmitted along a transmission medium and some of the signal power is reflected back to its origin, rather than being carried all the way along the cable to the other end. Reflection happens because of imperfections in the cable causing impedance mismatching and non-linear changes in the cable characteristics.
Crosstalk is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. High frequency crosstalk is crosstalk that occurs at high frequency transmissions.
A high-speed transceiver is made up of a high-speed transmitter (TX) and a high-speed receiver (RX), at opposite ends of a transmission medium, such as a backplane. A major consideration for high-speed transceiver design is crosstalk. Crosstalk results from parasitic inductance and capacitance coupling among the connectors, the printed circuit boards (PCB), the package, and the die. A package is the physical enclosure of a silicon chip that projects the connections to the actual chip in the form of pins exposed from the package. A die is the actual silicon on which the circuit is formed. Parasitic inductance and capacitance is impedance that is not taken into account when considering ideal circuit elements. This extra impedance usually has detrimental effects on the operation of circuits, reducing their bandwidth or enhancing their susceptivity to interference. Parasitic inductance and capacitance coupling is the parasitic impedance occurring among the components of a circuit. The amount of parasitic coupling is much stronger at higher frequencies than lower frequencies.
Similar to the insertion loss, crosstalk is frequency dependent. Insertion loss is the decrease in transmitted signal power resulting from the insertion of a device in a transmission channel and is usually expressed relative to the signal power delivered to that same part before insertion.
Crosstalk contributes to the noise portion of the signal-to-noise ratio (SNR) seen at the slicer. A slicer is a single bit analog to digital converter that converts an analog signal to a digital bit 1 when the analog signal passes a certain voltage threshold and to a digital bit 0 when the analog signal voltage falls below the voltage threshold. Signal-to-noise ratio is the ratio of a transmitted signal to the background noise of the transmission medium.
Low-pass filter is a frequency filter circuit that passes low frequency signals and blocks high frequency signals. Conversely, a high-pass filter is a frequency filter circuit passes high frequency signals and blocks low frequency signals. A band-pass filter is a frequency filter circuit that passes a band of frequencies in a frequency bandwidth, while blocking frequencies that fall outside that band.
Transmission channels have inherent frequency bandwidth limitations. Most transmission channels pass frequencies on the low end of the frequency bandwidth without trouble but attenuate high frequencies. In this respect, transmission channels act as low-pass filter. Equalization is a technique to boost the amplitude of high frequencies in the signal so that both low and high frequency content in a signal is preserved through the transmission channel. Equalization is also known as channel equalization. An equalization scheme is a specific equalization technique. An equalizer is a circuit that implements an equalization scheme. A linear equalizer uses only the signal in the forward direction through a transmission channel to perform the equalization. A decision feedback equalizer makes an equalization decision based on feedback of not just the current bit received through the transmission channel but also that of certain previous bits that have previously been received through the transmission channel.
A decision feedback equalizer (DFE) can differentiate between the signal and the crosstalk and can provide equalization without crosstalk enhancement, but the decision feedback equalizer (DFE) does not suppress crosstalk. The amount of crosstalk must be mitigated for reliable link operation represented by high bit-error-rate (BER) rate. A transmitter (TX), a transmission channel, and a receiver (RX) form a transmission link. A link is a transmission link, and the operation of the transmission link is called the link operation. This need for mitigation is especially true for high-speed applications. A bandwidth filter can be applied to increase the signal-to-noise ratio (SNR) by suppressing the crosstalk above the signal-band through its stop-band attenuation. A bandwidth filter is a device that passes frequencies within a certain frequency range or bandwidth, and rejects or attenuates frequencies outside that range. An example of an analog electronic band-pass filter is a resistor-inductor-capacitor (RLC) circuit. These filters can also be created by combining a low-pass filter with a high-pass filter. A signal-band is the frequency range of the valid signal. A stop-band is the frequency range outside of the signal-band. Stop-band attenuation is the rejection of frequencies in the stop-band.
In one currently used technique, adaptive bandwidth controller has been applied between the receiver linear equalizer and the decision feedback equalizer (DFE) to mitigate the amount of crosstalk for hybrid linear and decision feedback equalization in the receiver. However, the adaptive bandwidth controller is problematic in that it couples the linear equalizer and the decision feedback equalizer (DFE). A cleaner way for crosstalk control between the linear equalizer and the decision feedback equalizer (DFE) would therefore be desirable.
To adapt the transmitter pre and post emphasis filters the error information has to be communicated back to the transmitter. A second currently used technique achieves simultaneous forward and back-channels by using the orthogonal property between the differential and common-mode signals to provide a separate pathway on the same physical channel. While in theory, the differential and common-mode signals are orthogonal, in practice, inevitable channel non-idealities lead to signal coupling between modes, causing signal integrity degradation in both domains. The common-mode back-channel transmission appear as noise to the differential receiver, degrading the signal integrity of the forward channel. The common-mode back-channel transmission also degrades the differential receiver's offset and sensitivity performance. As a result, the margin of the forward link degrades. To satisfy the tight noise budget in high performance applications, the back-channel signal has to be limited to a very small swing relative to the forward signal.
On the other hand, the strong differential signal severely degrades low-swing common-mode signaling through crosstalk, and through large common-mode noise that could be comparable to the common-mode signal swing. As a result, the common-mode signaling obtained is not reliable. Therefore, in using the second currently used technique, design trade-offs have to be made between forward and back-channels, which result in a performance hit at both links.
A typical decision feedback equalizer (DFE) has limited operating range in the signal amplitude that the decision feedback equalizer (DFE) can handle. Dynamic signal amplitude adjustment with environmental change is usually required to keep the decision feedback equalizer (DFE) working in the desired range. Some form of gain adaptation in the transmission link is required to keep the transmission relatively error-free over time. The second currently used technique achieves gain adaptation by adapting transmitter swing dynamically through back-channel. However, this approach demands frequent transmitter adjustment and it directly translates to the bandwidth requirement for the back-channel.
Furthermore, simultaneous forward and back-channel transmission on the same physical link is preferred as being the most independent and cost effective way for signal transmission through a physical link. However, when the forward and the back-channel share the same physical link, the forward and back-channels compete for the frequency band resources. The second prior art solves this issue by putting the back-channel and forward-channel signals into orthogonal relationship by using an elaborate common-mode and differential-mode transmission scheme.
However, this scheme suffers from drawbacks as described above. Additionally, changing transmitter signal amplitude in the middle of the transmission creates additional crosstalk on the neighboring channel because the transmitter signal is usually much larger than the receive signal, especially in the long-reach applications. This additional crosstalk is undesirable.